Prior art devices for measuring and feeding particulate solid materials suffer many disadvantages and do not achieve accurate metering. Much of the solids metering art has involved batch processes where measurement occurred by dumping bags full of material into a container. Continuously metering the feed of dry, solid particulate material has been attempted with single and double screw feeders, vibratory feeders and hoppers, and feedback systems such as weigh conveyors. These devices suffer many inaccuracies from flow variations affected by relative humidity, depth of material in a hopper, variations in batches of material, extent of preconditioning of material, clogging, bridging, rat-holing, etc. Some materials can pack between the lands of a screw feeder and rotate around with the screw without advancing, and materials can accumulate and lodge in many places in many different ways. Any such accumulation of material in a feeder reduces the feed rate as the material collects and produces a surge in the feed rate whenever the accumulation breaks loose and joins the output flow. Feed rates varying with the depth of material in a supply hopper are especially troublesome, and variations in solid feed rates can occur rapidly over such a range that feedback systems cannot make the flow accurate, because of the necessary time lag involved. Mixing, stirring, and agitating devices have been used to condition material so it will feed more accurately, and these also have been only partially successful. In addition to other disadvantages, double screw feeders are easily disabled by contaminant objects that can catch between the screws and require shutdown and repair. Metering solids feeders have been arranged in tandem for successively smoothing out irregularities in the flow, with incomplete success, and high accuracy and reliability in continuously metering the feed of solid particulate material is not attainable for many materials by using devices presently on the market.
The invention involves recognition of a way of solving many of the problems involved in accurate metering of the continuous feed of solid particulate materials, and the invention aims at solving these problems for a wide variety of solid particulate materials with a relatively simple and inexpensive device. The invention also aims at accommodating extremely uneven supply rates; convenient, reliable, and economical operation; and wide variation in metering rates without loss of accuracy.